Precision machining requirements occurring during the precision working of hardened gears extend today from mass production to individual piece manufacturing. While methods and machines exist in the precision working of unhardened gears, which methods and machines are established and capable of high production in each individual case, this is not true to the same extent as for precision working of hardened gears.
Especially for the smallest lot sizes having the highest requirements for exactness and flexibility in the method and with the machine, we are aware of only one machine type on the market today with which it is possible to correct the topography of the tooth flanks (German OS No. 23 07 493 corresponding to U.S. Pat. Nos. 3,906,677 and 3,986,305). Due to the number of operating and adjusting axes, however, the machine requires a considerable amount of time in implementing change over procedures.
The known grinding method is based on a specific rolling position of the tool with respect to the workpiece tooth. This actually fixed rolling position is changed only for the purpose of correction. Two corrective influences are to be considered here, one being a profile correction depending on the rolling and the second being a flank-line correction depending on the grinding stroke in the longitudinal direction of the tooth. The rolling and the feed in the longitudinal direction of the tooth are constant in the adjusted case. This principle has a disadvantage, namely, that the action onto a concrete surface point can occur only "hastily". A goal-oriented operation is therefore not possible in the case of gears coated with a hard material.
A tooth-flank grinding machine operating according to the known generating method is described in German Patent No. 20 50 946. The workpiece is stationarily mounted in this machine and carries out only a rolling movement reduced to a pure rotary movement, whereas the translatoric portion of the rolling movement is associated with the grinding wheel. Because of the continuously occurring rolling movement, it is hardly possible to act onto precise concrete surface points in order to achieve a desired tooth-flank correction.
Therefore, the basic purpose of the invention is to provide a method with which it is possible to very precisely machine gears and to be able to work in a simple manner corrections into the surface of the tooth flanks. Furthermore, the basic purpose includes the provision of a machine having the least possible number of operating and adjusting axes and on which machining can be done according to the aforementioned method and which can be changed over quickly and simply.
This purpose is attained and made possible by the inventively important and novel separation of the movements: the workpiece is axially and radially stationarily mounted and carries out only a rolling movement about its axis of rotation, that is, a pure rotary movement, however, a rolling movement is not carried out during a working stroke of the grinding wheel. The grinding wheel yields, so to speak, during each rolling movement of the workpiece following the desired involute or flank shaping, that is, the grinding wheel moves up the tooth with the aforementioned "yielding" thereby providing a more advantageous movement. If helically toothed gears are ground, grinding occurs from one axial side of the workpiece tooth to the other along helixes, for which purpose the workpiece undergoes a suitable rotary movement during each grinding stroke.
The inventive method can be carried out with a cup-shaped grinding wheel, with which a point contact between the grinding wheel and the tooth flank is theoretically obtained. In practice, however, contact occurs over a narrow sickle-shaped surface. This surface can be reduced, when a bevel grinding wheel is used and still further with a grinding wheel having a concave work surface. Such a small touching or contact surface coming at least close to a point contact is a condition enabling gears, needing corrections in the surface of the tooth flanks, to be ground. It is, however, also important, in order to produce the necessary surface pressure during grinding, if a cubic boron nitride (CBN) coated wheel is used, because among others the small contact surface is important for this case.
Thus a very important characteristic of the novel method and novel machine is that the rolling step, namely, the rotating of the workpiece, and the rolling stroke, namely, the feeding of the grinding wheel in the Z-direction, can occur independently from one another. A whole series of advantages are connected with this and, which are not offered by the method and the machine according to German Patent No. 20 50 946:
When the rolling step and the rolling stroke are in a fixed relationship to one another, exact involutes are then produced, just like in the case of a fixed gear train or through roll cams and roll bands, however, step-by-step, for which reason the method can be identified as discontinuous. PA1 If the relationship is changed, however, remains the same during machining a tooth, then involutes are created which belong to a smaller or larger base circle. Thus these are angle of mesh corrections. PA1 If the relationship is changed during machining of each tooth, then tooth-profile corrections are created in the form of surface crownings or addendum and/or dedendum alterations. PA1 (a) the rolling cutting is a manufacturing method using a "stationary" tool, the inventive pitch-rolling-grinding is a (precision) machining method with a rotating tool; PA1 (b) during rolling cutting, each feed movement is followed by a non-work producing return stroke in the opposite direction, whereas in the inventive method, each feed movement in one direction is followed by a feed movement in the other direction; PA1 (c) during rolling cutting, the workpiece is guided along the tool with a step-by-step rotation, whereas in the inventive method the workpiece is stationary and the tool, namely the grinding wheel, is moved through a rolling stroke following a corresponding movement through respective rolling step.
Also during the known gear shaping with generating motion according to the generating method, a rolling movement is not carried out during the operating stroke (Maag-Taschenbuch, second edition, 1985, Page 318 under d). However, not insignificant differences exist between this method and the inventive method: